The present invention relates generally to the field of wind turbine aerodynamics, in particular to the optimizing of the energy output from a wind turbine rotor consisting of a plurality of blades.
Typically, the aerodynamic surfaces of wind turbine blades have sharp or moderately blunt trailing edges from which the wake is shed. The shedding of the wake and the confluence of flow from the pressure and suction sides of the profile are sources of aerodynamic noise, and increased drag and reduced lift.
For a wind turbine rotor of a given diameter it is possible to determine the total mechanical power available at a given wind speed. The efficiency of the wind turbine rotor at a given wind speed is the ratio between the shaft power and the total mechanical power available in the wind at that wind speed.
For everyday purposes the efficiency of a wind turbine rotor is displayed in the so-called power curve of the wind turbine. A power curve is a graph and/or a table of the electrical power output from the wind turbine as a function of the wind speed. For blade efficiency evaluations in absolute terms a mechanical power curve is necessary, since the efficiency of the wind turbine rotor at a given wind speed is the ratio between the shaft power and the total mechanical power available. The measurement of a mechanical power curve is difficult, however, requiring stable torque instrumentation on the rotating shaft of the turbine. Consequently, the rotor efficiency is commonly evaluated on the basis of the electrical power, thereby inherently including the effects of the losses in gearbox, generator and cables. For blade efficiency evaluations in relative terms this is sufficiently accurate, provided the gearbox, generator and cable losses are known and are kept unchanged during any modifications of the wind turbine rotor.
For the wind speed range that contributes most of the annual energy output the efficiency of a wind turbine rotor is a function of the ratio between lift and drag on the aerodynamic profiles of the blade. A high lift/drag ratio is preferred.
Certain aerodynamic profiles have shapes that provide lift/drag ratios above normal. These so-called laminar profiles are used e.g. on gliders that benefit from high lift/drag ratios. On wind turbines such profiles are not suitable, however, since they are very sensitive to surface contamination by e.g. insects or rain. Once contaminated their lift/drag ration drops to or below that of normal profiles.
Lift modifying devices have been used on wind turbine blades to improve the lift/drag ratio or otherwise adjust the aerodynamic characteristics of the blade. Common devices include stall strips, vortex generators and Gurney flaps. Generally, such devices have negative effects on drag, and trade-offs between lift and drag are normally expected.
Serrated trailing edges are known to improve the lift and drag characteristics of a lifting surface. Various embodiments are described in U.S. Pat. No. 5,088,665.
Serrated trailing edges are known to be used on wind turbines for noise reduction purposes. In such applications the serrations are usually limited to the outboard 10–20 percent of the span. An embodiment for noise reduction with hexagonal cross section of the serrations is described in U.S. Pat. No. 5,533,865.
A more general description of the use of serrated trailing edges on wind turbine blades with the purpose of noise reduction is presented in “Noise Reduction By Using Serrated Trailing Edges/K. A. Braun, N. v. d. Borg, T. Dassen, A. Gordner, R. Parchen”, Proceedings of EWEC 97, Dublin 1997.
None of the above descriptions or uses of serrated trailing edges have been shown to improve wind turbine rotor efficiency, or have been used to improve or attempt to improve wind turbine rotor efficiency.